Many different types of data storage media have been developed to store information. Traditional data storage media, for instance, include magnetic media, optical media, magneto-optic media, capacitive media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, capacitive, or magnetic changes on the surface of the media. For this reason, medium surface area generally poses physical limits on data densities of traditional media.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data is stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same medium volume using multiplexing techniques. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information, e.g., bit maps, can be stored as optical interference patterns within a photosensitive holographic recording material. This is done by intersecting two coherent laser beams within the photosensitive material. The first laser beam, called the object beam, is encoded with the information to be stored. The second laser beam, called the reference beam, interferes with the encoded object beam to create an interference pattern that is stored in the holographic recording material as a hologram.
A spatial light modulator device is typically used to encode the information into the object beam for holographic recording. A spatial light modulator device may include a set of optical elements that affect input light in order to encode a bit map in the object beam. For example, a reflective spatial light modulator device may include a set of specular mirrors that are individually controlled to define bits in the bit map. Alternatively, a transmissive spatial light modulator device may include a set of elements that can be made transmissive or opaque in order to either pass or block light and thereby define the bits of the bit map. In either case, when the object beam illuminates the spatial light modulator device, the spatial light modulator device can encode the information into the object beam. The object beam is then made to interfere with a reference beam to record a hologram in the medium.
When a stored hologram is later illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram interference pattern. Moreover, the diffracted light can be directed to reconstruct the original object beam. Thus, by illuminating a recorded hologram with the reference beam only, the data encoded in the object beam can be reconstructed and detected by a data detector such as a camera or other image capture device. In this manner, information stored in a recorded hologram can be read from a holographic medium.
One problem in holographic recording systems is referred to as “zero-order burning.” In particular, when Fourier transformed holograms are recorded, light intensity near the center of the object beam may significantly exceed light intensity near the edges of the object beam. For example, following a Fourier transformation of a data encoded object beam, the zero-order Fourier component of the holographic beams manifests this undesirable light intensity. The zero-order Fourier component of the holographic beam carries only information relating to the average intensity of the beam, and carries substantially none of the information encoded in the beam. The significant light intensity of the zero-order Fourier component near the center of the object beam can cause significant and undesirable exposure to the holographic medium at the Fourier transform plane, and can negatively impact the storage capacity of holographic media.